1. Field of the Invention
The invention relates to the field of power supplies, and in particular, to power supplies having battery back-up.
2. Background Information
Power supplies having back-up battery circuitry are known, such as uninterruptible power supplies (UPS's). In some power supplies having battery back-up, the batteries are always on-line, that is connected to the power supply output, and these are referred to as continuous battery back-up power supplies. In another variety of power supply, the batteries are switched to be on-line only when they are needed, and these are referred to as switched battery back-up power supplies. The present invention is directed to the switched back-up power supply variety.
In a known type of switched back-up power supply, the alternating current (AC) input to the power supply, also called the AC mains, is monitored/sensed with an AC sensor to determine when to switch the supply to run on the back-up batteries. However, there are many times when the AC monitoring/sensing is either too sensitive or not sensitive enough. These sensitivity problems result in either the batteries being brought on-line when they are not really needed, or not being brought on-line in time to avoid a negative consequence to the electronic equipment they support, e.g., too late to avoid a computer system crash.
If the sensitivity of the AC sensor is too high, the batteries may be brought on-line in response to power line noise when there is no real threat of an outage. In noisy installations, this can cause frequent unnecessary switching to back-up battery power, which may be wasteful of the energy stored in the batteries and could result in discharged batteries when a real back-up situation is present thereby causing a computer crash and/or result in other negative consequences over time to the battery back-up circuitry.
On the other hand, if the sensitivity of the AC sensor is too low, a substantial rapid AC power drop could occur without the back-up batteries coming on-line soon enough to avoid a substantial reduction in power supply output. Such a sudden power supply output drop could adversely affect sensitive electronic equipment, and in the case of computer systems, it could cause a computer system crash.
Therefore, a need exists for a better method of, and apparatus for, detecting when to bring the battery back-up circuits on-line reliably, so that battery energy is not wasted by bringing the battery back-up on-line when the batteries are not needed, but so that battery back-up is brought on-line when it is needed and in time to avoid a negative consequence, such as a computer system crash.
Some past attempts at solving this sensitivity problem have resulted in more expensive and complex AC sensing circuitry, while still generally not achieving optimum results. There is still room for improvement in determining when to bring battery back-up on-line since errors still frequently occur. Because of this, prudent design currently errs on the side of being over-sensitive in critical applications, such as for computer system back-up, to rule-out an occurrence of a loss of power. This tends to make such battery back-up systems expensive to operate and maintain, because the back-up batteries experience unnecessary recharge cycles, which can substantially shorten their useful life.
As is also known in the art, power supplies used with data processing equipment, for example, often require power factor correction (PFC) circuits to meet AC line harmonic requirements. As is known in the art, maximum power is delivered to a load when the voltage and the current are in phase. In circuits where the voltage/current is changing in magnitude, i.e., in AC circuits and pulsing DC circuits, such as power supplies, and where there is an inductive or capacitive load, the peak voltage will either lead or lag behind the current with respect to phase, and therefore less than 100% of the possible instantaneous power will be delivered to the load. A numerical value called the power factor relates the actual power delivered to the load to 100%. The power factor is thus the ratio of the actual power to 100%. In power supplies without power factor correction, a relatively low power factor of 0.7 is typical. Power factor correction circuitry attempts to get the power factor as close to 1.0 as possible.
As is known in the art, a capacitive load can be tuned to increase the instantaneous power delivered by adding an inductance, and an inductive load by adding a capacitance, for a given frequency. Power supplies typically provide a rectified AC (pulsed DC) voltage to a large storage capacitance which soon charges to the peak voltage of the AC input to the rectifier. Thereafter, the rectifier diodes only conduct when the AC input reaches approximately its peak values due to the reverse bias effect of the storage capacitance and the capacitance's slow discharge rate. The output of the diodes thus appears as a series of pulses rather than the desired ideal sinusoid.
In power circuits, the fundamental frequency of the AC input to the power supply is a nominal 60 Hz (in the U.S.). A full wave rectifier outputs sinusoidal pulses of DC with twice this fundamental frequency, i.e., 120 Hz. However, the situation is complicated by the fact that for a variety of reasons, there are generally a number of significant harmonics of the fundamental frequency on the power line and produced by the power supply itself. A line transformer is often used to electrically isolate the power supply from the AC line, and the transformer reduces the harmonics from the line.
Due to the large storage capacitance used in the power supply, the rectifier diodes only conduct during a part of the AC cycle near the peaks. Therefore, the diode current appears to be more like a series of short pulses, and as is known in the art, this produces more harmonics. Also, power line and load impedance can change at any time, for example, as the devices which make up the load are connected and disconnected from the power supply, e.g., turn on and off. In a computer system, for example, when a floppy drive is accessed, its motor is turned on, changing the load on the power supply.
In power supplies having a large storage capacitance, an inductance, sometimes referred to as a choke coil, is often provided to help to tune the load on the rectifier, reduce harmonics caused by the rectifier, and increase the power factor by bringing the current closer in phase to the voltage. However, additional measures, i.e., power factor correction circuits, are used to increase the power factor and cure the problem of AC line harmonics. Since the harmonics are out of phase with the fundamental frequency during part of their cycles, at least part of the energy otherwise available is lost.
As already mentioned, power factor correction circuits are known. An example of a power factor correction circuit is described and shown in FIG. 1 herein corresponding to FIG. 2 of U.S. Pat. No. 5,737,204 by Alan E. Brown. In the power supply 200 of FIG. 1, a power factor correction (PFC) circuit includes an inductance (L), a diode (D1) and a controlled switch (Q) disposed between a bulk capacitor (C) and a full-wave bridge rectifier 104. The switch (Q) is controlled so that the voltage (VDC2) at the capacitor (C) is regulated to a be at a predetermined level, and so that the PFC circuit appears to be a purely resistive load, i.e., so that the current and voltage at the PFC input are essentially in phase.
The switching provides a regulated DC voltage by switching current through the diode (D1) and inductor (L) into the capacitance (C). The capacitance (C) is coupled to output a DC voltage to a main power converter 108 as its load.
In the power supply 200 of the Brown patent shown herein in FIG. 1, a battery back-up interface circuit (BBIC) is switched to be connected to the input of the power factor correction circuitry (PFC) when a drop in the PFC circuit voltage output (VDC2) is detected with voltage monitor 202. The power factor correction circuit (PFC) now also acts as a boost circuit to boost the battery (B) voltage (VDC1) provided to the PFC circuit to the appropriate level (VDC2) at the capacitance (C). The boosting is achieved by switching the input voltage on and off through switch (Q) by pulse width modulation control 106 and by virtue of the inductance (L) and capacitance (C) being a tuned circuit, i.e., operating as a "chopper."
The FIG. 1 battery back-up system of Brown has only one PFC front-end and power converter 108. As just described, the battery back-up is brought on-line by sensing the voltage output of the boost section of the PFC circuit, and turning-on the battery switch (S) FET with the control circuit (110) when this output voltage reaches a predetermined low level. The signal interfacing the battery back-up switch control 110 to the PFC front-end power supply voltage monitor 202 is labeled "ON" in FIG. 1.
However, although U.S. Pat. No. 5,737,204 of Brown has shown a method of interfacing a DC battery back-up to a single PFC power supply front end in response to an AC outage condition, as described above, a need exists for a battery back-up system for supporting a plurality of PFC front-end power supplies to a single battery back-up.
Further, a need exists for a battery back-up system to handle the case where one (or more) of the power supplies is defective and the respective PFC boost circuit(s) does not work properly. In such a situation, a need exists for controlling the battery back-up so that the battery back-up does not turn on, or if already on turns off, for this condition.
Further, in Brown's circuit shown herein in FIG. 1, detecting the restoration of main AC power 102 is done by sensing the voltage across a diode (D2) with a voltage detector 204, and controlling the battery switch FET (S) to disconnect the battery (B). In an alternative embodiment of Brown (see Brown patent FIG. 1), one or more missing cycles of AC will be detected with a detection circuit 112 which then causes switching on of the battery back-up circuitry. In that embodiment, restoration of AC is determined by a return of one or more AC cycles detected by the detection circuit 112 which then switches the battery back-up off.
However, in either embodiment of Brown, Brown only detects a complete loss/return of AC power with the AC cycle detection circuit 112. Such a circuit cannot detect a brown-out condition, i.e., a condition where the AC line voltage is present but reduced to a level where circuit malfunction could occur, e.g., below about 5% of the nominal AC line voltage.
Therefore, a need exists for an alternative method of turning the battery back-up ON when AC input is lost or reduced by a significant amount, and OFF when AC has been restored and/or returned to a nominal threshold voltage level.
Therefore, a need exists for an improved method of determining when to use battery back-up that achieves optimum results in solving the basic problem without requiring expensive and complex AC sensing circuitry.
In particular, there exists a need for an apparatus and method of controlling a single battery back-up system which can back-up a plurality of PFC power supplies; there exists a need for an apparatus and method of controlling a single battery back-up system which can detect a brownout condition and provide battery back-up; and there exists a need for an apparatus and method of controlling a single battery back-up system which can detect a PFC power supply failure and take appropriate action to disconnect the battery back-up from the failed power supply.